Apparatus intended, inter alia, for checking and controlling means of payment, e.g. currency, substitute currency and other credit instruments or documents are known to the art. An example of such apparatus is found, for instance, in Swedish Lay-Out Print No. 448037, in which the means of payment contains magnetic particles and is passed through a primary magnetic field. The presence of secondary or introduced magnetic fields induced by the presence of the primary magnetic field are detected separately, and an error signal is produced when the means of payment located in the primary magnetic field fails to introduce secondary magnetic fields.
The drawback with this known apparatus is that the effect of the detection of the secondary, introduced magnetic fields is liable to be impaired as a result of poor signal/noise relationships and because it is the absolute level of the introduced, secondary magnetic field which is to be detected.
The object of the present invention is, inter alia, to greatly reduce this risk and to integrate the primary magnetic field source and the detector in one and the same function.